Function and mechanism of LGR4 and LGR5 in modulation of Wnt signaling Abstract LGR4 and LGR5 are two related receptors of the rhodopsin-like 7TM receptor superfamily. LGR5 is specifically expressed in the rapidly cycling crypt stem cells of the entire gastrointestinal tract and hair follicle stem cells in the bulge whie LGR4 is essential for survival of the crypt stem cells. Complete knockout of either LGR4 or LGR5 in the mouse leads total embryonic/neonatal lethality. Among the hundreds of 7TM receptors in the rhodopsin family, LGR4 and LGR5 are among the few that are essential for development. Just recently, we discovered that LGR4/5 function as ligands of R-spondins, a group of secreted proteins with vital functions in embryonic development, se determination, nail formation and growth of crypt stem cells. Activation of LGR4/5 by R-spondins were shown to robustly potentiate Wnt/b-catenin signaling, a system that is well known to be essential for development and for survival of stem cells. The discovery of R-spondins as ligand of LGR4/5 provides a molecular basis for stem cell-specific effect of the Wnt/b-catenin pathway and for the pleiotropic functions LGR4/5 and R- spondins has in development and stem cell survival. However, the exact functions and signaling mechanisms of the RSPO-LGR4/5 ligand-receptor system remain unknown. No evidence of LGR4/5 coupling to heterotrimeric G proteins or to b-arrestin was found. Thus LGR4/5 may have unique signal transducers and mechanisms that relay activation of LGR4/5 to increased Wnt/b-catenin signaling and thus differentiate them from hundreds of other rhodopsin-like receptors. In this proposal, we plan to identify and characterize the transducers and mechanisms of LGR4 and LGR5 using a variety of approaches. Understanding of the mechanism of LGR4 and LGR5 will provide important insights not only to the operating principles of stem cells, the molecular processes of sex determination and organ development but also to the general field of signal transduction.